In many of the international safety standards, there are restrictions on output voltages of driving circuits in lighting devices with light emitting diodes (LEDs). Those international safety standards include, for example, IEC 61347-2-13 and UL1310, etc. In IEC 61347-2-13, it's prescribed that output voltages of LED drivers can satisfy the SELV (safety extra-low voltage) requirements only under a condition of the output voltages ≦50V AC RMS or the output voltages ≦120V DC. In UL1310, it's prescribed that output voltages of LED drivers can satisfy the SELV requirements only under a condition of the output voltages ≦42.4V AC RMS or the output voltages ≦60V DC.
However, in some applications, a large number of LEDs are needed so as to satisfy the illumination requirements. In this situation, a plurality of LED clusters need to be connected in parallel with each other, so as to avoid output voltages thereof from going beyond the restrictions of the safety standards. However, the usage of too many LED clusters may cause too many related accessories to be used in the whole system, such that the system is made more complicated, and the cost thereof is greatly increased.
FIG. 1 illustrates a conventional LED lighting device using a plurality of LED clusters, wherein the current balancing cells and their protection circuits on the right of the secondary side of a transformer Tr in an AC power supply generator are all integrated to the side of a LED panel, and the left side circuits are all integrated to the side of the AC power supply generator. The power generated by the AC power supply generator is an alternating current (AC) power to be applied to the accessories and circuits of the lighting device on the right of the secondary side of the transformer Tr. In some cases, since a greater number of LEDs need to be connected in every cluster to satisfy the illumination requirements, the output voltage of the AC current source (namely, the voltage on the secondary output terminal of the transformer) rises higher than a voltage satisfying the SELV requirements. Namely, the output voltage is higher than the upper limit of the safety extra-low voltage prescribed by IEC 61347-2-13 or UL1310, etc.
Therefore, the conventional way to solve the problem is to reduce the number of LEDs in every cluster and increase the number of clusters connected in parallel, such that the output voltage of the AC current source can be reduced to meet the SELV requirements, as shown in FIG. 2. However, this may increase the number of the accessories such as filter circuits, protection circuits needed in every LED cluster. Moreover, during a process of standard production, sometimes the number of LEDs in every cluster must keep identity, which may further multiply increase the number of LED clusters connected in parallel together with the number of accessories in every LED cluster, such that the entire cost of the lighting device is greatly increased.
Accordingly, it's a problem that needs to be solved urgently with the lighting device having voltage reduction function to be developed to overcome the above deficiencies in the prior art and to meet the SELV requirements with less cost.